1. Field of the Invention
The present invention relates to a liquid crystal molecule and to a liquid crystal display device and a liquid crystal optical spatial modulation device each using the liquid crystal molecule.
2. Description of the Related Art
In recent years, a liquid crystal display device (LCD) of an active matrix driving type such as a TFT (thin film transistor) has been widely used over the range of from small mobile uses to large television sets.
In the LCD, techniques such as impulse driving have been being adopted, thereby attempting to realize a high response speed. However, it is the actual situation that the LCD is inferior to a plasma display (PDP), a field emission display (FED) and the like in display quality regarding dynamic images due to, for example, dynamic image blur to be caused by a slow response speed of a liquid crystal material itself.
Meanwhile, attempts have also been being made to increase a speed of the current 60 Hz frame rate drive to 120 Hz or 240 Hz (high frame rate drive), thereby enhancing the dynamic image display quality. Though the dynamic image display quality in the LCD partly depends upon the driving system including a TFT, it depends mostly on the response characteristics of the liquid crystal material itself. Namely, the foregoing problem may not radically be solved, and the high frame rate drive may not be realized, unless the liquid crystal material is responsible to a high speed.
In the light of the above, it is the situation that a liquid crystal material which can cope with the high frame rate drive and which can exhibit high-speed response permitting realization of a high dynamic image display quality is keenly demanded.
As a liquid crystal capable of realizing high-speed response, a nematic liquid crystal attended by a flexoelectric effect, a ferroelectric liquid crystal, an antiferroelectric liquid crystal and the like are known. The present inventors have paid attention to an electroclinic effect in a smectic A phase.
The electroclinic effect is a phenomenon in which in a smectic A phase, optical axes of liquid crystal molecules uniaxially aligned (liquid crystal molecule longitudinal axes) are inclined according to an intensity of an impressed electric field (see Garoff, et al., Physical Review Letters, Vol. 38, 1977, page 848). When this type of cell is disposed between orthogonal polarizing plates, a transmitted light quantity according to an angle (tilt angle) formed by an optical axis of the polarizing plate and an optical axis of the liquid crystal is obtained (see the following expression (A)), and a maximum transmittance is obtained at a tilt angle of ±45°.T/T0=sin2(2θ)×sin2(nΔnd/λ)  (A)
Here, T represents a transmitted light quantity; T0 represents an incident liquid quantity; θ represents an angle (tilt angle) formed an optical axis of the polarizing plate and an optical axis of the liquid crystal; Δn represents a birefringence of the liquid crystal; d represents a thickness of the liquid crystal layer; and λ represents a wavelength of the transmitted light.
Results obtained by calculating the dependency of the transmittance on the tilt angle in case of a retardation (=Δnd) giving a maximum transmittance according to the foregoing expression (A) are shown in FIG. 6.
A response time in the electroclinic effect is very fast as from several μseconds to several tens μseconds. Also, there is a merit that in a low electric field intensity, an inclination angle (tilt angle) of the optical axis is proportional (i.e., voltage modulation of transmitted light is possible). That is, it may be said that this is a display mode which is very suited for the active matrix driving.
However, the tilt angle which has hitherto been revealed in liquid crystal materials is not so large that sufficient optical modulation is not obtainable yet.
As a liquid crystal material displaying a large tilt angle, a material system in which a siloxane is added to a non-chiral end is known. A reason for this may be considered to reside in the fact that such a siloxane has a larger volume than that of usual alkyl chains, and when a flexible functional group is added to a terminal group, a core portion which contributes to the optical modulation becomes easily movable by the action of an electric field. According to Naciri, et al., Chem. Mater., 1995, 7, pages 1397 to 1402, in case of a liquid crystal molecule having a structure in which a siloxane is added to a non-chiral end, a maximum tilt angle of 26° is obtained. However, according to the foregoing expression (A), the transmittance is about 60% at the most. Also, since a nitro group is introduced as a polarizing group, the polarization is large. Thus, such a liquid crystal material is still insufficient in consideration of putting a display device or the like into practical use.
Furthermore, JP-A-2008-150334 discloses a liquid crystal molecule having a structure having an asymmetric carbon atom and having a core portion including a plurality of aromatic groups and an organosiloxane at positions opposite to each other interposing the asymmetric carbon atom therebetween. However, this liquid crystal molecule is not sufficiently satisfactory in view of the fact that a tilt angle is small, and a more increase of the tilt angle is desirable.